“A properly designed and operated Heating, Ventilating, and Air Conditioning (HVAC) system can provide a slightly higher pressure in the building relative to the soil gas pressure and the outdoor air pressure and thereby minimize soil gas entry and control ventilation to improve overall indoor air quality.”
Reference: Designs for New Residential HVAC Systems to Achieve Radon and Other Soil Gas Reduction, 1993 International Radon Conference, by Timothy M. Dyess, Chief of the Radon Mitigation Branch, U.S. Environmental Protection Agency, Air & Energy Engineering Research Laboratory Research Triangle Park, NC.
The United States EPA recommends sealing air leaks in the shells of buildings to reduce energy costs, allow for improved environmental control, and minimize the amount of outdoor air needed to maintain a slight positive indoor air pressure. “Measurements in existing schools show that a slight positive air pressure equal to the pressure of as little as 0.001 inches of water column (.25 Pascals) relative to sub-slab and outdoor air pressure, reduces indoor radon levels by preventing radon entry.”
Reference: Radon Prevention in the Design and Construction of Schools and Other Large Buildings, January 1993, US Environmental Protection Agency, Office of Research and Development, Washington DC 20460, EPA/625/R-92/016, p 25.
“The most common way that radon enters a home is when lower indoor air pressure draws air from the soil, bedrock or drainage system into the house. If there is radon in the soil gas, it will also be drawn in. Just as gravity will make water flow from a high elevation to a lower elevation, pressure differences will make radon-laden soil gasses move from an area of higher pressure to an area of lower pressure. If cracks, holes, and pores in the foundation are open to the soil, radon will be drawn indoors. In an average home, about 90% of the radon originates in the soil.”
Reference: Radon-Resistant Construction Techniques for New Residential Construction, U.S. Environmental Protection Agency, Offices of Research and Development and Air and Radiation, Washington, DC 20460, EPA/625/2-91/032, February 1991, page 3-5.
“The heating, ventilating and air conditioning (HVAC) systems in schools have been shown to impact radon levels, either by depressurizing areas and increasing radon entry from the soil, or by pressurizing and ventilating to reduce indoor radon concentrations. An HVAC system approach was the preferred radon reduction technique over soil depressurization in 23 of the 26 schools evaluated.”
Reference: HVAC Systems in the Current Stock of US K-12 Schools, U.S. Environmental Protection Agency, EPA-600/R-92/125.
“Radon mitigation by pressurization counteracts the negative pressure effects of exhaust fans and natural convection by increasing the flow of mechanically-supplied outdoor air into the building (or, in some buildings, by reducing the amount of general exhaust). A pressure sensor, located away from exterior walls to minimize wind effects, measures the indoor/subslab pressure differential. The pressure sensor provides information to the control system, which regulates the flow of outdoor air.”
Reference: Reducing Radon in Schools: A Team Approach, US Environmental Protection Agency, Office of Air and Radiation, EPA 402-R-94-008, April 1994.
“Plan the HVAC systems so that the building interior in all ground contact rooms is slightly pressurized (about 1 Pascal or 0.004 inches of water column) relative to subslab and outdoor pressure) .”
Reference: Radon Prevention in the Design and Construction of Schools and Other large Buildings. January 1993, US Environmental Protection Agency, Office of Research and Development, Washington DC 20460, EPA/625/R-92/016, p. 26.
“The use of barrier techniques as a stand-alone system is not recommended, but it is recommended that some amount of effort be made to limit the entry of radon through the foundation. This can be done by using:
Foundation materials with low vapor permeability
Sealants at cracks
Sealants or gaskets at joints and penetrations
Foundation coatings, normally used for damp-proofing
Membranes surrounding the foundation
NOTE: It should be pointed out that attempts to control radon by making a gas-tight barrier around the foundation have not been completely effective. It is likely they have done some good, but many newly constructed buildings that relied on barriers as the only radon reduction technique have elevated levels of indoor radon.”
Reference: Radon-Resistant Construction Techniques for New Residential Construction, US Environmental Protection Agency, Offices of Research and Development and Air and Radiation, Washington, DC 20460, EPA/625/2-91/032, February 1991, page 6.
Figure 1: Radon reduction due to sealing of openings.
Radon gas can enter through the pores and very small cracks in concrete floors and walls.
Excessive positive or negative
pressures can move large quantities of moisture-laden air through
openings in a building's shell, leading to condensation and mold growth within building cavities. Moist insulation is
less efficient. Building
experts have observed pressure-derived moisture problems at only ± 1 Pascal
(enough
to raise a column of water 0.004 inches) during severe humidity conditions."
— John Krigger,
Residential
Energy (3rd Edition), page 75.
Moisture condensation and damage can occur below the roofs and within outer walls of air-conditioned buildings if indoor air pressure is significantly less than outdoor air pressure. When the temperature of materials in the wall and ceiling cavities is below the dew point temperature of moist air leaking in, moisture will condense, enabling growth of fungi that damage materials and produce noxious gasses.
Moisture condensation and damage can occur below the roofs and within outer walls of heated buildings if indoor air pressure is significantly greater than outdoor air pressure. When the temperature of materials in the wall and ceiling cavities is below the dew point temperature of of moist air leaking out, moisture will condense, enabling growth of fungi that damage materials and produce noxious gasses.
ANSI/ASHRAE Standard 62.2-2004, Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings, is the only nationally recognized indoor air quality standard developed solely for residences. It recommends proper pressurization of buildings. Maintaining a small positive pressure, relative to the outdoors and the soil gas under the building, limits the entrance of moisture from outdoors and from the soil. To prevent mold and mildew formation in a building, a very common strategy is to keep indoor air pressure slightly higher than outdoor air pressure and soil gas pressure.
The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) develops national consensus standards. Their Internet address is www.ASHRAE.org
